Embodiments of the present invention are directed to a method for isolating mammalian stem cells from a population of cells by immunolabeling the population of cells with MHC/HLA gene-product markers. Further embodiments of the present invention are directed to the purification and enrichment of cells so isolated.
Nearly every cell in an animal""s body, from neural to blood to bone, owes its existence to a stem cell. A stem cell is commonly defined as a cell that (i) is capable of renewing itself; and (ii) can give rise to more than one type of cell (that is, a differentiated cell) through asymmetric cell division. F. M. Watt and B. L. M. Hogan, xe2x80x9cOut of Eden: Stem Cells and Their Niches,xe2x80x9d Science, 284, 1427-1430 (2000). Stem cells give rise to a type of stem cell called progenitor cells; progenitor cells, in turn, proliferate into the differentiated cells that populate the body.
The prior art describes the development, from stem cell to differentiated cells, of various tissues throughout the body. U.S. Pat. No. 5,811,301, for example, the disclosure of which is hereby incorporated by reference, describes the process of hematopoiesis, the development of the various cells that comprise blood. The process begins with what may be a pluripotent stem cell, a cell that can give rise to every cell of an organism (there is only one cell that exhibits greater developmental plasticity than a pluripotent stem cell; this is a fertilized ovum, a single, totipotent stem cell that can give rise to an entire organism when implanted into the uterus). The pluripotent stem cell gives rise to a myeloid stem cell. Certain maturation-promoting polypeptides cause the myeloid stem cell to differentiate into precursor cells, which in turn differentiate into various progenitor cells. It is the progenitor cells that proliferate into the various lymphocytes, neutrophils, macrophages, and other cells that comprise blood tissue of the body.
This description of hematopoiesis is vastly incomplete, of course: biology has yet to determine a complete lineage for all the cells of the blood (e.g., it is has yet to identify all the precursor cells between the myeloid stem cell and the progenitor cells to which it gives rise), and it has yet to determine precisely how or why the myeloid cell differentiates into progenitor cells. Even so, hematopoiesis is particularly well studied; even less is known of the development of other organ systems. With respect to the brain and its development, for example, U.S. Pat. No. 6,040,180, the disclosure of which is hereby incorporated by reference, describes the xe2x80x9ccurrent lack of understanding of histogenesis during brain development.xe2x80x9d U.S. Pat. No. 5,849,553, the disclosure of which is hereby also incorporated by reference, describes the xe2x80x9cuncertainty in the art concerning the development potential of neural crest cells.xe2x80x9d
The identification and isolation of stem cells has daunted researchers for decades. To date, no one has identified an individual neural stem cell or hematopoietic stem cell. F. H. Gage, xe2x80x9cMammalian Neural Stem Cells,xe2x80x9d Science, 287, 1433-1488 (2000). There are two principal difficulties. First, stem cells are rare. In bone marrow, for example, where hematopoiesis occurs, there is only one stem cell for every several billion bone marrow cells. G. Vogel, xe2x80x9cCan Old Cells Learn New Tricks?xe2x80x9d Science, 287, 1418-1419 (2000). Second, and more importantly, researchers have heretofore been unable to identify molecular markers which are unique to stem cells; to the typical immunoassay, most stem cells look like any other cell. Id. Compounding this problem is that primitive stem cells may be in a quiescent state. As a result, they may express few molecular markers. F. H. Gage, supra.
A method to identify and effectively isolate stem cells would be of immense importance. Researchers are already transplanting immature neurons, presumed to contain neural stem cells, from human fetuses to adult patients with neurodegenerative disease. The procedure has reduced symptoms by up to 50% in patients with Parkinson""s disease in one study. M. Barinaga, xe2x80x9cFetal Neuron Grafts Pave the Way for Stem Cell Therapies,xe2x80x9d Science, 287, 1421-1422 (2000). Many of the shortcomings of this procedure, including the ethical and practical difficulties of using material derived from fetuses, could be addressed by using cultures of isolated stem cells, or stem cells obtained from adult individuals.
Transplantation of purified hematopoietic stem cells could replace bone marrow transplants. Such stem cells are free of contamination with cancer cells, and could induce life-long tolerance of donor organ or tissue transplants. I. L. Weissman, xe2x80x9cTranslating Stem and Progenitor Cell Biology to the Clinic: Barriers and Opportunities,xe2x80x9d Science, 287, 1442-1446 (2000). Transplantation of islet stem cells could replace insulin therapy; skeletal muscle stem cell transplants could treat patients with life-threatening muscle loss, such as can occur with muscular dystrophy; and transplantation of heart muscle stem cells could be used to regenerate heart muscle ravaged by myocardial infarction. Id. There are many more important uses, but one must first have the starting materialxe2x80x94the stem cells that make these uses possiblexe2x80x94before they may be investigated and applied. The invention described herein is the first to provide that starting material.